There is an image reading device that conveys a sheet of document and that reads an image on the sheet at a reading position in a conveyance path by using an image reading unit. The image reading devices of this type include a device that detects, on a read image, positions at which a front end and rear end of the sheet is positioned in the conveying direction, and that uses results of the detection to determine an image size of the sheet in the read image, for example.